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PageMidtermMidterm Be ready to do calculations using the following concepts:Be ready to do calculations using the following concepts: –Kinetic and potential energy –Newton’s and Kepler’s laws of gravitation –Radiation (Wavelength/Frequency relation, photon energy, Wien and Stefan-Boltzmann’s law, Doppler shift) –Diffraction limit, Small-angle formula Be ready to discuss solar system formation, extrasolar planetsBe ready to discuss solar system formation, extrasolar planets Formulas will be given on the exam, but you need to know how to use themFormulas will be given on the exam, but you need to know how to use them BRING YOUR SCIENTIFIC CALCULATOR!BRING YOUR SCIENTIFIC CALCULATOR!

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Page Outline of lecture Almost 500 planet candidates have now been observed around other starsAlmost 500 planet candidates have now been observed around other stars –How have they been detected? –What do they look like? –What do they tell us? –What does the future hold? With thanks to Susan Cartwright Simulation by Geoff Bryden, JPL: Solar system and disk based on that observed around the star Gl 876 Please remind me to take a break at 12:45 pm

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Page The Main Points The ~ 500 planets we have detected to date are only a sub-set of potential planets out thereThe ~ 500 planets we have detected to date are only a sub-set of potential planets out there These new solar systems have raised big questions about how our own Solar System formedThese new solar systems have raised big questions about how our own Solar System formed Future search methods have high probability of finding more (and more varied) planetsFuture search methods have high probability of finding more (and more varied) planets It’s hard to find Earth-like planetsIt’s hard to find Earth-like planets –But prospect of finding Earth-like planets is thrilling!

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Page Main Points, continued Planet formation and solar system evolution are in midst of a “paradigm shift”Planet formation and solar system evolution are in midst of a “paradigm shift” –Prevailing ideas of 15 years ago don’t work any more, in light of new data –Lots of ferment, discussion, computer simulations –Ultimately will confront data from other solar systems of varying ages A VERY EXCITING TIME!A VERY EXCITING TIME! Exoplanet EncyclopaediaExoplanet Encyclopaedia

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Page The ancient Greek “atomists” argued that there are other solar systems Leucippus ( B.C.)Leucippus ( B.C.) The worlds come into being as follows: many bodies of all sorts and shapes move from the infinite into a great void; they come together there and produce a single whirl, in which, colliding with one another and revolving in all manner of ways, they begin to separate like to like.The worlds come into being as follows: many bodies of all sorts and shapes move from the infinite into a great void; they come together there and produce a single whirl, in which, colliding with one another and revolving in all manner of ways, they begin to separate like to like. Epicurus ( B.C.)Epicurus ( B.C.) There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number, as was already proven, … there nowhere exists an obstacle to the infinite number of worlds.There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number, as was already proven, … there nowhere exists an obstacle to the infinite number of worlds. Unfortunately, the atomists were overshadowed by Aristotle ( B.C.) who believed thatUnfortunately, the atomists were overshadowed by Aristotle ( B.C.) who believed that There cannot be more worlds than one.There cannot be more worlds than one.

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Page Dangerous to believe in plurality of worlds! Giordano Bruno, “On the Infinite Universe and Worlds”“This space we declare to be infinite; since neither reason, convenience, possibility, sense-perception nor nature assign to it a limit. In it are an infinity of worlds of the same kind as our own...” Giordano Bruno, “On the Infinite Universe and Worlds” Unfortunately, plurality of worlds was a heretical idea. Bruno was burned at the stake in 1600!Unfortunately, plurality of worlds was a heretical idea. Bruno was burned at the stake in 1600!

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Page Why is it so hard to find planets around other stars? Faint planet glimmer is lost in glare from parent starFaint planet glimmer is lost in glare from parent star –Planets are small, close to their parent star, and shine by reflected starlight Thought experiment:Thought experiment: –Imagine grain of rice an inch from a 100 Watt light bulb. Someone standing at end of a long dark hall would see only the light bulb, not the grain of rice. Consider the case of Jupiter and the Sun:Consider the case of Jupiter and the Sun: –As seen from the nearest star, Alpha Centauri, Jupiter would appear a billionth as bright as the Sun. –Jupiter would also be extremely close to the Sun, only 4 arc sec away. Since all other stars are farther than Alpha Centauri, Jupiter would be even harder to detect from other starsSince all other stars are farther than Alpha Centauri, Jupiter would be even harder to detect from other stars

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Page Planets are very hard to observe directly (by detecting their own light) Planets are too faint compared with their starPlanets are too faint compared with their star –This brown dwarf star is barely visible - and its star is faint Planets shine by reflected lightPlanets shine by reflected light –Planets close to parent stars are brightest, but hardest to see (ground) (space)

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Planet Detection Indirect: measurements of stellar properties revealing the effects of orbiting planetsIndirect: measurements of stellar properties revealing the effects of orbiting planets –Most planets to date have been detected by indirect methods Direct: pictures or spectra of the planets themselvesDirect: pictures or spectra of the planets themselves –Only recently starting to be used successfully

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Gravitational Tugs The Sun and Jupiter orbit around their common center of mass.The Sun and Jupiter orbit around their common center of mass. The Sun therefore wobbles around that center of mass with same period as Jupiter.The Sun therefore wobbles around that center of mass with same period as Jupiter.

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Gravitational Tugs The Sun’s motion around the solar system’s center of mass depends on tugs from all the planets.The Sun’s motion around the solar system’s center of mass depends on tugs from all the planets. Astronomers around other stars that measured this motion could determine the masses and orbits of all the planets.Astronomers around other stars that measured this motion could determine the masses and orbits of all the planets.

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Astrometric Technique We can detect planets by measuring the change in a star’s position on sky.We can detect planets by measuring the change in a star’s position on sky. However, these tiny motions are very difficult to measure (~ arcsecond).However, these tiny motions are very difficult to measure (~ arcsecond).

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Doppler Technique Measuring a star’s Doppler shift can tell us its motion toward and away from us.Measuring a star’s Doppler shift can tell us its motion toward and away from us. Current techniques can measure motions as small as 1-2 m/s (walking speed!).Current techniques can measure motions as small as 1-2 m/s (walking speed!).

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First Extrasolar Planet: 51 Pegasi Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period.Doppler shifts of the star 51 Pegasi indirectly revealed a planet with 4-day orbital period. This short period means that the planet has a small orbital distance – well within orbit of Mercury.This short period means that the planet has a small orbital distance – well within orbit of Mercury. This was the first extrasolar planet to be discovered (1995).This was the first extrasolar planet to be discovered (1995). Insert TCP 6e Figure 13.4a unannotated

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Page 51 Peg b (1995) Half the mass of Jupiter, yet orbiting much closer to the Sun than Mercury!

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Other Extrasolar Planets Doppler shift data tell us about a planet’s mass and the shape of its orbit.Doppler shift data tell us about a planet’s mass and the shape of its orbit.

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Page Stellar wobble depends on mass, period and eccentricity of planet Size depends on mass of planet Periodicity depends on period of planet Low mass, high mass Small period, large period Shape depends on eccentricity of planet Circular, eccentric Planetary signatures

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Page Doppler shift Look for periodic shift in star’s spectrumLook for periodic shift in star’s spectrum –Does not depend on distance of star –Need massive planet near star »the closer the planet, the faster the orbital speed (of both planet and star) –Need very good spectrum »measure Doppler shifts of < 1 part in 1,000,000 90% of planet detections to date90% of planet detections to date Incredibly hard measurements have now become standardIncredibly hard measurements have now become standard

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Page Radial velocity method doesn’t give all the orbital information Doppler shift only detects velocity along line of sightDoppler shift only detects velocity along line of sight –Can’t distinguish massive planet (or brown dwarf!) in tilted orbit from less massive planet in edge- on orbit –They both have the same line-of-sight velocity The only way to resolve this ambiguity is to observe using another methodThe only way to resolve this ambiguity is to observe using another method WHY?

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Thought Question Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? A. It has a planet orbiting at less than 1 AU. B. It has a planet orbiting at greater than 1 AU. C. It has a planet orbiting at exactly 1 AU. D. It has a planet, but we do not have enough information to know its orbital distance.

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Thought Question Suppose you found a star with the same mass as the Sun moving back and forth with a period of 16 months. What could you conclude? A. It has a planet orbiting at less than 1 AU. B. It has a planet orbiting at greater than 1 AU. C. It has a planet orbiting at exactly 1 AU. D. It has a planet, but we do not have enough information to know its orbital distance.

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Page Find your own planet! “Systemic” console: you participate in finding signals of planets from telescope data that others have obtained“Systemic” console: you participate in finding signals of planets from telescope data that others have obtained

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Page ● From ground- based observatories, detect shifts > 1-2 m/sec ● The nearer to the star and the more massive the planet, the easier to detect Limitations of Doppler technique

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Page Otto Struve advocated these techniques in 1952! "An intrinsically improbable event may become highly probable if the number of events is very great.... [I]t is probable that a good many of the billions of planets in the Milky Way support intelligent forms of life.”

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PageTransitsTransits As planet moves across face of star, it blocks a tiny bit of starlightAs planet moves across face of star, it blocks a tiny bit of starlight Watch for periodic dimming of starWatch for periodic dimming of star

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Page Planet detected around the star HD by transit method Planet is 70% mass of Jupiter, but orbits in just 3.5 daysPlanet is 70% mass of Jupiter, but orbits in just 3.5 days So it is very close to its parent starSo it is very close to its parent star Thus far ~ 70 planets have been found this wayThus far ~ 70 planets have been found this way Amateur astronomers have organized to watch for fluctuations in star brightnessAmateur astronomers have organized to watch for fluctuations in star brightness ch.org/http://www.transitsear ch.org/http://www.transitsear ch.org/http://www.transitsear ch.org/

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Transits and Eclipses A transit is when a planet crosses in front of a star.A transit is when a planet crosses in front of a star. Eclipse is when a planet goes behind a starEclipse is when a planet goes behind a star From both, can learn about atmosphere of planet (beginning and end of transit, eclipse)From both, can learn about atmosphere of planet (beginning and end of transit, eclipse)

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Page Transiting planets Can measure size, mass, temperature and spectraCan measure size, mass, temperature and spectra Can test the atmospheric models that have been developed for planets in our solar systemCan test the atmospheric models that have been developed for planets in our solar system Some of these planets are subject to more exciting conditions than the ones in the Solar System:Some of these planets are subject to more exciting conditions than the ones in the Solar System: – Small distance from star – Extreme eccentricities

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NASA’s Kepler Space Mission to detect transiting planets March 2009: NASA mission Kepler was launchedMarch 2009: NASA mission Kepler was launched Scans a hundred thousand stars in the constellation Cygnus for planetsScans a hundred thousand stars in the constellation Cygnus for planets Measurement precision expected to detect and characterize Earth-sized planetsMeasurement precision expected to detect and characterize Earth-sized planets Kepler has already been able to detect the light from a known transiting extrasolar gas giant, HAT-P-7bKepler has already been able to detect the light from a known transiting extrasolar gas giant, HAT-P-7b Many discoveries to be announced soonMany discoveries to be announced soon

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Page Planet detection: fine structure on microlensing light curve Candidates for several planets have been discovered this wayCandidates for several planets have been discovered this way Potentially very useful: can detect planets at large distances from usPotentially very useful: can detect planets at large distances from us –Even farther away than transit method can –Much farther than radial velocity or astrometry can

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PageAstrometryAstrometry Look for star moving on the sky (with respect to other stars)Look for star moving on the sky (with respect to other stars) Need to measure angles (motion on sky) of < arc secondsNeed to measure angles (motion on sky) of < arc seconds No confirmed candidates yetNo confirmed candidates yet

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Page Sensitivity needed for astrometry detection of Jupiter around our Sun Sun’s mass is about 1000x Jupiter’s massSun’s mass is about 1000x Jupiter’s mass Astrometric accuracy needed:Astrometric accuracy needed: –Radius of Jupiter’s orbit around center of mass: 5.2 AU –Radius of Sun’s orbit around center of mass: AU From nearest star, Sun’s motion on the sky is like a penny seen from 600 km away!From nearest star, Sun’s motion on the sky is like a penny seen from 600 km away!

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Page Direct Imaging Use planet’s own lightUse planet’s own light Take image of itTake image of it –Can reconstruct full orbit by watching it go around Can also obtain spectraCan also obtain spectra –Learn about physical conditions, atmosphere, maybe even presence of life Jupiter is a billion times fainter than the Sun, in visible light!Jupiter is a billion times fainter than the Sun, in visible light!

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Page Glowing young planets This star has 3 orbiting planets - the first imaged planetary system! This star has 3 orbiting planets - the first imaged planetary system! Advanced observing techniques were used to block the star’s light Advanced observing techniques were used to block the star’s light Adaptive Optics was used to sharpen images Adaptive Optics was used to sharpen images Observations were repeated over years, confirming planetary motion Observations were repeated over years, confirming planetary motion The planets are young and hot, and therefore glow more brightly than by reflected starlight alone The planets are young and hot, and therefore glow more brightly than by reflected starlight alone This star has 3 orbiting planets - the first imaged planetary system! This star has 3 orbiting planets - the first imaged planetary system! Advanced observing techniques were used to block the star’s light Advanced observing techniques were used to block the star’s light Adaptive Optics was used to sharpen images Adaptive Optics was used to sharpen images Observations were repeated over years, confirming planetary motion Observations were repeated over years, confirming planetary motion The planets are young and hot, and therefore glow more brightly than by reflected starlight alone The planets are young and hot, and therefore glow more brightly than by reflected starlight alone Keck Observatory infrared image of star HR8799 and 3orbiting planets; orbital directions indicated by arrows. The light from the star was subtracted, but a lot of ‘noise’ remains. Three planets orbiting HR8799

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Page Hubble Space Telescope visible image of the star Fomalhaut (whose light was blocked), with a dust belt similar to the Kuiper belt. Inset: Images taken ~2 years apart show a planet moving around the star. Star location Neptune-sized orbit First images of exoplanets: Fomalhaut

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Page Upsilon Andromedae Two planets are several times more massive than JupiterTwo planets are several times more massive than Jupiter The third planet, mass 75% that of Jupiter, is so close to the star that it completes a full orbit every 4.6 Earth daysThe third planet, mass 75% that of Jupiter, is so close to the star that it completes a full orbit every 4.6 Earth days

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PageQuestionsQuestions Has the discovery of other solar systems changed your own feelings about the universe?Has the discovery of other solar systems changed your own feelings about the universe? Are you comfortable or uncomfortable with the idea that they are so different from our own?Are you comfortable or uncomfortable with the idea that they are so different from our own?

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Page Unanticipated characteristics of extra-solar planets Much higher eccentricity in most of their orbitsMuch higher eccentricity in most of their orbits Much higher fraction of planets very close to their parent starsMuch higher fraction of planets very close to their parent stars Many planets are “super-Jupiters” (up to 10 times more massive than Jupiter)Many planets are “super-Jupiters” (up to 10 times more massive than Jupiter)

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Page Eccentric Orbits

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Page Many extrasolar planets are very close to parent stars This is a selection effect (caused by detection method)This is a selection effect (caused by detection method) But it does show that such planets exist!But it does show that such planets exist! Our Solar System is very different (green points) - Mercury is farther away from Sun than many of the extra-solar planets are from their starsOur Solar System is very different (green points) - Mercury is farther away from Sun than many of the extra-solar planets are from their stars

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Page Characteristics of Extra-Solar Planets: Mass Many planets much more massive than JupiterMany planets much more massive than Jupiter Selection effect: we are missing planets on the low-mass end

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Page Patterns in eccentricity Most new planets are in elliptical orbitsMost new planets are in elliptical orbits Short period planets:Short period planets: –Very close to parent stars, very low eccentricity –Same process that moved planets close to star circularized their orbits

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Page What are these planets? Our Solar System has small rocky planets close to star, large gas giants further awayOur Solar System has small rocky planets close to star, large gas giants further away –no experience of large massive planets close to sun in our Solar System Theory of giant planet formation says they have to form outside “frost line”Theory of giant planet formation says they have to form outside “frost line”

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Page This is the “paradigm shift” Original theories of solar system formation developed when our own Solar System was the only oneOriginal theories of solar system formation developed when our own Solar System was the only one –Mostly circular orbits –Giant planets in outer solar system, terrestrial planets inside New Solar Systems are (in general) not like oursNew Solar Systems are (in general) not like ours Needs a new theoryNeeds a new theory How to arrive at a new paradigm?How to arrive at a new paradigm? –Mostly use computer simulations to develop ideas, test hypotheses, make predictions –Test predictions against observed young solar systems, disks

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Page Theories for how giant planets got so close to their stars 1. Interactions between individual new planets and gaseous disk. “Migration” 2. After gas disk cleared away, several giant planets in outer parts of solar system were left –Three-body gravitational interactions between them –One giant planet got slung outwards, a second was slung inwards and got “captured” by the star in a close orbit –But why isn’t the close orbit very elliptical? Why didn’t Jupiter migrate inwards close to Sun?Why didn’t Jupiter migrate inwards close to Sun?

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Planetary Migration in a massive disk A young planet’s motion can create waves in a planet- forming disk.A young planet’s motion can create waves in a planet- forming disk. Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.Models show that matter in these waves can tug on a planet, causing its orbit to migrate inward.

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Page 1. Planet formation in gaseous disk One planet in a gaseous diskOne planet in a gaseous disk Accretion begins, gap starts to formAccretion begins, gap starts to form Planet can continue to accrete mass even after a “gap” in disk has formedPlanet can continue to accrete mass even after a “gap” in disk has formed From computer simulation by Pawel CiecielagFrom computer simulation by Pawel Ciecielag

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Hypothesis 2: Gravitational Encounters Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit.Close gravitational encounters between two massive planets can eject one planet while flinging the other into a highly elliptical orbit. –Orbit would later have to be circularized (drag?) Multiple close encounters with smaller planetesimals can also cause inward migration.Multiple close encounters with smaller planetesimals can also cause inward migration.

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Orbital Resonances Resonances between planets can also cause their orbits to become more elliptical.Resonances between planets can also cause their orbits to become more elliptical.

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Thought Question What happens in a gravitational encounter that allows a planet’s orbit to move inward? A. It transfers energy and angular momentum to another object. B. The gravity of the other object forces the planet to move inward. C. It gains mass from the other object, causing its gravitational pull to become stronger.

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Thought Question What happens in a gravitational encounter that allows a planet’s orbit to move inward? A. It transfers energy and angular momentum to another object. B. The gravity of the other object forces the planet to move inward. C. It gains mass from the other object, causing its gravitational pull to become stronger.

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What have we learned? Using Doppler shift measurements we have detected ~ 500 planets around nearby starsUsing Doppler shift measurements we have detected ~ 500 planets around nearby stars –massive planets close to stars, often in eccentric orbits  not what was expected  may arise when Jupiter- like planets migrate inwards after formation –more likely to find planets around stars that have more heavy elements What does this imply?What does this imply? –does not imply that such systems are typical  detection method is biased –does imply that they are possible! –does not imply that systems like ours are uncommon  Jupiter is barely detectable –but does not provide evidence that they are common! No “other Earths” yet but we’re getting much closerNo “other Earths” yet but we’re getting much closer

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Page What is a Habitable Planet? Not too bigNot too big Not too smallNot too small Not too hot or too coldNot too hot or too cold A good planet is:

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Page 67 What does “habitable” mean to you? Right temperatureRight temperature Liquid waterLiquid water Free oxygen to breathFree oxygen to breath Light to keep you warm and to see byLight to keep you warm and to see by Radiation shieldRadiation shield Asteroid protection from atmosphere and JupiterAsteroid protection from atmosphere and Jupiter Next few slides courtesy of Stephen Kane

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Page 68 Things That Affect Temperature Want temperature so you can have liquid water on the surface of the planetWant temperature so you can have liquid water on the surface of the planet 1.Temperature of star 2.Distance from the star 3.Shape of planet’s orbit: circular or elliptical 4.Planet’s atmosphere: greenhouse gases

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Page 69 The Habitable Zone for Stars of Various Masses The Habitable Zone (HZ) in green is the distance from a star where liquid water is expected to exist on the planets surface.The Habitable Zone (HZ) in green is the distance from a star where liquid water is expected to exist on the planets surface.

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PageQuestionQuestion NASA has made its goal to find habitable planets. (Gas giants don’t “matter” as much….)NASA has made its goal to find habitable planets. (Gas giants don’t “matter” as much….) –Do you agree that this should be the main priority?

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Page The Main Points The ~ 500 planets we have detected to date are only a sub-set of potential planets out thereThe ~ 500 planets we have detected to date are only a sub-set of potential planets out there These new solar systems have raised big questions about how our own Solar System formedThese new solar systems have raised big questions about how our own Solar System formed Migration and planet-planet interactions play a role in reshaping a planetary systemMigration and planet-planet interactions play a role in reshaping a planetary system Future search methods have high probability of finding more (and more varied) planetsFuture search methods have high probability of finding more (and more varied) planets It’s hard to find Earth-like planetsIt’s hard to find Earth-like planets –But prospect of finding Earth-like planets is thrilling!

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Page Main Points, continued Planet formation and solar system evolution are in midst of a “paradigm shift”Planet formation and solar system evolution are in midst of a “paradigm shift” –Prevailing ideas of 10 years ago don’t work any more, in light of new data –Lots of ferment, discussion, computer simulations –Ultimately will confront data from other solar systems of varying ages A VERY EXCITING TIME!A VERY EXCITING TIME! Planet formation and solar system evolution are in midst of a “paradigm shift”Planet formation and solar system evolution are in midst of a “paradigm shift” –Prevailing ideas of 10 years ago don’t work any more, in light of new data –Lots of ferment, discussion, computer simulations –Ultimately will confront data from other solar systems of varying ages A VERY EXCITING TIME!A VERY EXCITING TIME!